riskywhiskey

My cats were clear the O2 was messed up from the factory motor having a crack in the head and causing issues before I purchased the vehicle with a brand new crate motor and trans so it's had a few things that should have been but were not replaced such as sensors it's not always the cat typically cats are a life of vehicle non service but with the condition of the rust on the exhaust and my old code reader not picking up the variations in the O2 sensor I thought it was the cat but the engine temp and back pressure ratio being normal left me confused

dave1123

Question; how do you measure back pressure ratio?

riskywhiskey

You have a specific pressure that is released from the exhaust valve not sure how exactly the dealer measured it but they said it is within spec

dave1123

Yeah, I'm aware of what it is, but IDK how you measure it. Also, is it a ratio of exhaust pressure compared to atmospheric or pressure difference above and below the cats and/or muffler?

riskywhiskey

It's below the cats but before the muffler my brother explained to me because he's done it a few times that they use a compression tester at the downstream 02 socket and the motor is hand cranked through each cylinder

dave1123

Okay....so that's a static test for system restrictions, I guess. Being downstream I don't know what it proves. I'd be curious of a dynamic test comparing upstream and downstream pressures to determine possible cat blockage.

dave1123

The reason I'm asking this question is my engine burns a little oil but it seems to be just at idle and the tailpipe smokes just a little only then. The exhaust smells weird then, but the engine doesn't seem to lack power and I get good fuel mileage. My muffler has a loose baffle that rattles and I thought it might be the big downstream cat, but we cut it off and inspected it. It's perfectly clean and not loose so we welded it back on. That cat is not monitored at all. IDK about the condition of the upstream cats.

I need to install an oil catch can in the CCV system to see if that helps with my oil problem.

riskywhiskey

im not entirely sure i was up for 22 hours when he was explaining this to me( will get a better explanation after sleep) but i have an idea of your rattle i had a rattle chatter chirp going on sounded like a bad muffler when i was sitting idle or in gear going slow i noticed the tail pipe was just touching the heat shield for the gas tank i stuck a prybar in the hanger in the back and pushed the tail pipe towards the outside of the vehicle and tada the chirp rattle was gone

dave1123

Nope. Mine's inside the muffler. Up on the lift, you can bang on the muffler with the side of your fist and it rattles. It's a cheap Soundmaster that's 2 years old.

CRLAMBERTJR

I like your thinking !!! The 3.73s are what I was thinking. Factory or Canyon wheels 15x8 with 31x10.50x15s and 3" lift. I found a great link to compare them side by side!!! http://www.grimmjeeper.com/gears.html With the 3.73s would be even better mileage with 31s. Less RPM at most speeds ! I try to keep mine under 2100. I'm really concerned about the fuel economy too. You said #24 injectors ,,, 4-hole Bosch??? Anyone tried the 12- hole models ? Going with a banks header and 2.5 back with new hi-flo cat and muffler. Not sure which muffler ? Really like the wheels Cruiser linked. I been watching a set in charcoal on ebay,,, $361 shipped !!! Would rather have the silver ones. I also keep looking at the American Racing AR23s. Really like these !!!

97grand4.0

oops, old thread.

You can put a federal cat on in NYS even if it is a CAL system. Thats why they sell them at AZ and advanced. Ask me how I know. You might need to change the 02 downstream sensors. However if you take it to one of the big rip off places they will tell you you cannot do that and try to sell you the 900$ job.
About a buck for a fed converter.